Biological filtration is routinely used in applications like sewage and waste water management and in the keeping of tropical and cold water fish by hobbyists, etc. Biological filtration relies on the activity of specific bacteria cultured in the filter to break down toxic waste products into less harmful substances. A good filter material should provide the largest surface area for bacteria to colonize but shall not block the fluid flow. Unfortunately, filters with large surface area (like porous media, foams or sponges) tend to clog up easily, thus reducing the effectiveness of the filter. Moreover, due to the distribution of the size of flow passages, fluid will tend to flow through only a few large passages, thus greatly reduces the effectiveness of the entire filter medium. The filter design discussed in this patent provides uniform size of flow passages thus the fluid flow through the entire filter instead of a few passages, giving the filter a large effective area for the bacteria to colonize. Flow through parallel plates also offers one of the best geometries for low flow resistance in fluid dynamic, thus reducing the chance of clogging. It can be used for both wet/dry and submerged biological filtration systems.